1. Field of the Invention
The present invention relates to a method for fabricating a microstructure array, a method for fabricating a mold or a master of a mold (in the specification the term xe2x80x9cmoldxe2x80x9d is chiefly used in a broad sense including both a mold and a master of a mold) for forming a microstructure array, a method for fabricating a microstructure array using the mold, and a microstructure array. This invention particularly relates to a mold for forming a microlens array, a method for fabricating the mold, and a method for fabricating the microlens array using the mold.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 or 3 microns to about 200 or 300 microns and an approximately semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, optical receivers and inter-fiber connections in optical communication systems.
Meanwhile, earnest developments have been made with respect to a surface emitting laser and the like which can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been repeatedly decreased in size as semiconductor processing techniques have been developed and advanced. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing.
In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in the fields of optical information processing, such as optical parallel processing-operations, and optical interconnections. Furthermore, active or self-radiating type display devices, such as electroluminescence (EL) panels, have been enthusiastically studied and developed, and a highly-defined and highly-luminous display has been proposed. In such a display, there is a heightened desire for a microlens array which can be produced at a relatively low cost and with a large area, as well as with a small lens size and a large NA.
There are presently a number of prior art methods for fabricating microlenses.
In a prior art microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20 (1) L51-54, 1981), a refractive index is raised at plural places in a substrate of multi-component glass. A plurality of lenses are thus formed places with a high-refractive index. In this method, however, the lens diameter cannot be large, compared with the intervals between lenses. Hence, it is difficult to design a lens with a large NA. Further, the fabrication of a large-area microlens array is not easy since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array. Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens quality, such as a focal length, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions. Furthermore, since a thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a misfit between their thermal expansion coefficients as an integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, a difficulty in joining or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another prior art microlens-array fabrication method using a resist reflow (or melting) method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p23-34, 1991), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warp and so forth, in contrast with the ion exchange method.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of resin, wetting conditions between the substrate and resin, and the heating temperature. Therefore, variations between lots are likely to occur while fabrication reproducibility per a single substrate surface is high.
Further, when adjacent lenses are brought into contact with each other due to the reflow, a desired lens profile cannot be secured due to the surface tension. Accordingly, it is difficult to achieve a high light-condensing efficiency by bringing the adjacent lenses into contact and decreasing an unused area between the lenses. Furthermore, when a lens diameter from about 20 or 30 microns to about 200 or 300 microns is desired, the thickness of deposited resin must be large enough to obtain a spherical surface by the reflow. It is, however, difficult to uniformly and thickly deposit the resin material having desired optical characteristics (such as refractive index and optical transmissivity). Thus, it is difficult to produce a microlens with a large curvature and a relatively large diameter.
In another prior art method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1 (1989)-261601), or a wet etching method (see Japanese Patent Application Laid-Open No. 5 (1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warp due to the difference in the thermal expansion coefficient can be solved, in contrast with the ion exchange method.
In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large investment in equipment is needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm2 (100 cm-square) because the electron beam impact area is limited.
Further, in the wet etching method, since an isotropic etching using a chemical action is principally employed, an etching of the metal plate into a desired profile cannot be achieved if the composition and crystalline structure of the metal plate vary even slightly. In addition, etching will continue unless the plate is washed immediately after a desired shape is obtained. When a minute microlens is to be formed, a deviation of the shape from the desired one is possible due to etching lasting during a period from the time a desired profile is reached to the time the microlens is reached.
Further, there also exists a mold fabrication method using an electroplating technique (see Japanese Patent Application Laid-Open No. 6 (1994)-27302). In this method, an insulating film having a conductive layer formed on one surface thereof and an opening is used, the electroplating is performed with the conductive layer acting as a cathode, and a protruding portion acting as a mother mold for a lens is formed on a surface of the insulating film. The process of fabricating the mold by this method is simple, and cost is reduced. Similar such methods are also disclosed in Japanese Patent Application Laid-Open No. 8 (1996)-258051 and Japanese Patent Publication No. 64 (1989)-10169.
The problem occurring when a plated layer is formed in an opening by the electroplating technique will be described by reference to FIGS. 1A and 1B. FIGS. 1A and 1B illustrate a radius variation or distribution of plated layers 105 formed in a two-dimensional array on a substrate 101. In the above fabrication method using electroplating in an electroplating bath, a distribution or variation of an electroplating-current density occurs over the substrate 101 due to a pattern of the openings (i.e., the electrode pattern) formed in an insulating mask layer 103 to expose an electrode layer 102. More specifically, the electric field is unevenly concentrated (stronger in a peripheral region than in a central region), and the electroplating growth is hence promoted near the periphery of the pattern of the arrayed openings. As a result, there is a distribution or variation of the size of semispherical microstructures 105 on the substrate. Therefore, when this substrate is used as a mold for forming a microlens array, the specifications of respective microlenses vary over the array.
An object of the present invention is to provide a method for fabricating a microstructure array (typically a microlens array such as a semispherical microlens array, a flyeye lens and a lenticular lens) with good performance and a reduced size distribution of microstructures flexibly, readily and stably; a method for fabricating a mold for forming a microstructure array; a fabrication method for a microstructure array using the mold, and so forth.
The present invention is generally directed to a method for fabricating an array of microstructures which includes the following steps:
preparing a substrate with an electrically-conductive portion;
forming an insulating mask layer on the electrically-conductive portion;
forming a plurality of openings in the insulating mask layer to expose the electrically-conductive portion;
forming a first plated or electrodeposited layer in the opening and on the insulating mask layer by electroplating or electrodeposition; and
forming a second plated layer on the first plated or electrodeposited layer and on the electrically-conductive portion by electroless plating to reduce a size distribution of microstructures over the array.
In the above fabrication method, the first plated layer is formed by electroplating, or the first electrodeposited layer is formed by electrodeposition using an electrodepositable organic compound. The array pattern is typically a two-dimensional array pattern which is periodical in at least a direction, a two-dimensional array pattern which is periodical in four mutually-orthogonal directions, or a periodical stripe pattern. In terms of an influence of the pattern of the conductive portion exposed through the openings on the distribution of a current density at the time of the electroplating or electrodeposition, the array pattern may create a current distribution in which the current density is uneven over the array. Typically, the opening has a circular shape and the microstructure is a semispherical microstructure, or the opening has an elongated stripe shape and the microstructure is a semicylindrical microstructure.
More specifically, the following constructions are possible based on the above fundamental construction.
The second plated layer is formed by electroless plating using an electroless plating solution containing a reducing agent of hypophosphite such as sodium hypophosphite. Thereby, corrosion resistance and wear resistance of the microstructure array can be improved.
The fabrication method may further include a step of forming a third plated layer on the second plated layer by electroplating, or a step of forming a third plated layer on the second plated layer by electroless plating which preferably uses an electroless plating solution containing a reducing agent of hypophosphite. Thereby, corrosion resistance and wear resistance of the microstructure array can be improved.
The third plated layers may be continuously formed at their bottom portions. A flyeye lens can be fabricated by using the microstructure array fabricated by such a method.
In the fabrication method, the first plated or electrodeposited layer and the second plated layer (additionally, the third plated layer) may be formed such that a horizontal bottom diameter or width of a semispherical or semicylindrical microstructure is approximately in a range from 1 xcexcm to 200 xcexcm. Such a minute microlens array is strongly desired with accurate size, good controllability and high stability, and the fabrication method of this invention can meet this desire.
In the fabrication method, the first plated or electrodeposited layer and the second plated layer (additionally, the third plated layer) may be formed such that a distribution of horizontal bottom diameters or widths of semispherical or semicylindrical microstructures (in this specification, the distribution is used as a ratio of a difference between a maximum value and a minimum value relative to the minimum value concerning the size of microstructures) is approximately less than 20%. When the size distribution takes such a value, the microstructure array, such as a mold for forming a microlens array, is of practical use.
In the fabrication method, the first plated or electrodeposited layer may be formed such that a ratio of a horizontal bottom diameter or width of the first plated or electrodeposited layer relative to a horizontal bottom diameter or width of a semispherical or semicylindrical microstructures is approximately less than 0.5. When such a condition is satisfied, a satisfactory size distribution can be readily achieved. As the thickness of the electroless plated layer in a vertical direction increases relative to the entire thickness or radius in the vertical direction of the microstructure, the size distribution of the microstructures decreases. Therefore, in order to better achieve a small distribution, a thickness ratio of the electroless plated layer relative to the entire microstructure is preferably as large as possible. On the other hand, the process speed of the electroless plating is lower than that of the electroplating or electrodeposition. The above ratio is determined considering the above factors.
In the fabrication method, the first plated or electrodeposited layer may be formed such that a diameter or width of the first plated or electrodeposited layer is approximately less than 10 xcexcm. Thereby, the microstructure array with microstructures having a bottom diameter or width approximately in a range from 1 xcexcm to 200 xcexcm can be readily achieved with a preferable distribution.
The fabrication method can further include a step of forming a mold on the substrate with the first plated or electrodeposited layer and the second plated layer (additionally, the third plated layer) by, for example, electroplating, and a step of separating the mold from the substrate. Thereby, a mold for forming a microstructure array such as a microlens array can be fabricated.
The fabrication method can further include a step of coating the substrate having the first plated or electrodeposited layer and the second plated layer (additionally, the third plated layer) with a first resin, a step of hardening the first resin, a step of separating the first resin from the substrate, and a step of coating the hardened first resin with a second resin having a refractive index different from a refractive index of the first resin. Thereby, a preferable microlens array can be fabricated.
The present invention is also directed to a microstructure array including the following:
a substrate having an electrically-conductive portion;
an insulating mask layer formed on the electrically-conductive portion, in which a plurality of openings are formed to expose the electrically-conductive portion;
a first plated or electrodeposited layer formed in the opening and on the insulating mask layer by electroplating or electrodeposition; and
a second plated layer formed on the first plated or electrodeposited layer and on the electrically-conductive portion by electroless plating.
Also in this microstructure array, the above specific structures may be adopted. The microstructure array is typically a mold for forming a microlens array, a lenticular lens or a flyeye lens.